1. Field of the Invention
The present invention relates to digital input devices and, more particularly, to the control of exposure therein.
2. Description of the Related Art
Although the light source in a digital input device such as a scanner is more or less constant, papers reflect light to a varying degree. L* is a colorimetric quantity that correlates well with the perceived lightness of a paper. This measure is scaled so that the perfect reflecting diffuser has value 100; one unit corresponds to a just noticeable difference (jnd). The table below shows the L* values for some typical papers:
______________________________________ Medium L* ______________________________________ perfect reflecting diffuser 100 premium paper 97 recycled copier paper 95 photographic AgX paper 93 European recycled paper 87 ______________________________________
The table indicates that the perceived lightness difference of typical papers spans a range of 10 jnd units. In a linear intensity space where sensors operate, such as the reflectance Y, the range is from 94 for the premium paper to 71 for the European recycled paper.
The range of values present in the original image or document can be called the tone gamut. Typically the lowest value in the gamut corresponds to black ink and the highest value corresponds to the paper background. The range of values for which a unique signal is generated by the lamp and sensor system in a scanner can be called its dynamic range.
A system is usually designed so that its dynamic range contains an original with the darkest ink on the lightest paper. Because of fluorescent substances often used in paper, the lightest value can be larger than the value of the perfect reflecting diffuser. This dynamic range can be called the worst case.
When the image is output on a display monitor or a printer, this device will also be calibrated to the worst case, i.e., the lightest possible value will be used to represent the worst case lightest pixels. In a system that reproduces lightness relative to the perfect reflective diffuser, an image scanned from a dark background such as a photograph will be reproduced by using ink to make the background darker (or the signal to a display monitor will be reduced) because the value without ink (or full display monitor signal) will be reserved to the worst case.
The human visual system (HVS) already adjusts for the background or surround. This is why lightness (i.e., brightness relative to an area perceived to be white) is a more important perceptual attribute than brightness for a gray-scale and color reproduction. If a reproduction system does not map the input paper lightness into the output paper lightness, the appearance of the facsimile will be judged as inferior (e.g., smudgy) by a human observer.
In the patent literature the adjustment for different papers, or more generally backgrounds, is sometimes called shading correction and is sometimes called exposure enhancement. This should not be confused with a compensation for sensor artifacts such as photo response non-uniformity (PRNU) and dark current, which in the literature is called normalization.
FIG. 1 is a flow chart illustrating a prior art method of processing a scan line. A document (100) containing an image is scanned (110) and then normalized (120). Exposure control (shading correction) is then performed (130) prior to color transformation (140) and data compression (150). The resulting information is then sent to the scan line buffer 170 or further application dependent processing is performed (160).
The standard solution adopted in many desktop scanner applications and digital copiers is to perform a first preview scan at a low resolution. The so obtained image is analyzed to determine the paper color of the original (or background value) and other parameters. The values so obtained are then used to set the scanner controls, after which the final scan is performed.
In applications such as sheet-fed scanners and color facsimile, it is not possible to perform such a preview scan. In particular, in the case of a color facsimile machine, the image is transmitted during the scan and concomitantly printed on a remote machine, precluding any form of post-processing.
A solution sometimes used is to assume a worst case or a typical case. If the assumption errs towards a high lightness, the scanned image will not have a white background, yielding a smudgy appearance, decreasing the performance of image compression algorithms, and consuming more ink or toner when the image is printed. If the assumption errs towards a low lightness, detail will be lost in the highlights of the image, yielding a washed out appearance.
Manufacturers of digital copiers and color facsimile machines have encountered this problem and developed various approaches. Previous attempts to solve this problem are mostly ad hoc. The disadvantages of methods obtained by trial and error are that they are not robust and cannot easily be adapted when the technology improves non-linearly.
Thus, it can be seen that single-scan constraints impose exposure control limits upon current technology digital input devices and hinder the use of these devices in many applications.
Therefore, there is an unresolved need for an exposure control mechanism that will significantly improve alignment of tone gamut to the dynamic range of a single-scan digital input device while compensating for such phenomena as the change of the response of the sensor due to thermal effect during slow scans.